Structure Placement mode: Now uses official "Placement Models" where applicable; Passability and Footprint Validity checks are now 100% accurate and use the same per-grid-square Placement check as the internal game code; Placement Grid visualization is "height-locked" to the structure ghost height; Structure Ghost movement is more snappy (instantaneous, no "blend");
Bug fix: Terran Bunker + Neosteel upgrade was not saving units occupying slots 5 & 6
